Parasite software is software that has been installed on a user's computer, typically without the user's knowledge, to perform tasks on behalf of another individual or entity. Examples of parasite software include spyware and adware. Often, the objective of the parasite software is not to harm the user's computer, but rather to provide information to the parasite software's owner and/or to consume free computer resources.
Unlike viruses and worms that palpably and malevolently affect a computer, parasite software often goes unnoticed on users' personal computers. Many anti-virus software packages installed on users' personal computers do not detect the presence of parasite software. Thus, a user may take corrective action to remove parasite software only when he/she either notices performance issues with his/her personal computer or learns that the parasite software exists on another user's computer.